I found this method by Luiska on the BBC forum website, it worked a treat and led to a lot less stressful Christmas day! I had served it with chunky ovenbaked chips, which I guess some people would see a travesty, but it was yum! I used onions instead of mushrooms as I'm allergic to mushrooms...
Fillet or sirloin beef
2 chopped onions
2 packets ready made puff pastry..
Fry up the onions until caramelised, and when cold ( this is really important for food hygiene)., mix with the pate. then follow the method below...
Sear the outside of the beef & then put it on a rack to cool and drain. Roll puff pastry, cut a piece somewhat bigger than fillet, prick with fork and bake until crisp & golden. Cool. Spread cooked pastry with half paté-onion mix, top with fillet spread with remainder, cut off excess cooked pastry corners & cover with raw pastry sheet, tucking edges underneath, making sure that the tuck-in is fairly minimal or it gets stodgy. Decorate with pastry strips or leaves or whatever, but don't glaze. Refrigerate overnight. Bring to room temp somewhere dry, make a couple of steam holes, then glaze with egg wash and bake.